1. Field of the Invention
The instant invention relates to a chart recorder for printing electrical signals on paper, and more specifically to a chart recorder for printing electrocardiogram (ECG) signals on paper having precisely positioned graphic lines preprinted on the paper to form a grid pattern, which chart recorder automatically loads the paper into the recorder, has a drag inducing system to impart drag to the paper to hold it precisely in position within the recorder for precise printing and includes an odometer system for determining the amount of paper passing through the recorder.
2. Description of Related Art
Many recorders exist for recording ECG signals on paper. Typically, these recorders pass paper between a print roller and a print head so that the print head prints a representation of the ECG signal on the paper. Often the paper used in these recorders has preprinted grid lines so that reference to certain points of the printed ECG signal may be more easily made.
The Association for the Advancement of Medical Instrumentation (AAMI), the professional organization setting standards in the medical profession for recording ECG signals, has set standards regarding alignment of the preprinted grid lines on the printing paper with the printed ECG signal. The AAMI standard is that no more than one half millimeter of misalignment is allowed along the length of the printing device which is typically a thermal print head. This standard approximates the goal of having a straight vertical line printed by the printing device on the paper be exactly parallel to the vertical grid lines of the paper. This allows identical amplitudes of an ECG signal to be printed on the paper parallel to the vertical grid lines as shown in FIG. 1A.
However, often the paper becomes misaligned within the recorder. This misalignment causes the printed ECG signal to be misaligned with the preprinted grid lines as shown in FIG. 1B. This misalignment may be the result of improper loading, a lack of precision formed and fitted pieces within the recorder, or the lack of proper positioning or alignment of the paper within the chart recorder.
The lack of proper positioning of the paper within the recorder often occurs because of a lack of tension on the paper in the recorder. The lack of tension on the paper allows the paper to pull or bind within the recorder in response to movement of or uneven contact with the feeding mechanism of the recorder or as a result of uneven contact with the printing device. This problem is exacerbated by misalignment within the feeding system itself so that parts of the paper are pushed or pulled in greater or lesser amounts as compared to other parts of the paper. In addition, parts of the paper may be caught or bound within the recorder. The pushing, pulling or binding causes lateral forces on the paper as it moves through the recorder. These lateral forces move the paper sideways or cause it to twist within the chart recorder. This sideways or twisting movement causes the paper to be misaligned with the print head.
Another contributing cause of misalignment of paper within the recorder, as shown in FIG. 3, is due to the passage of folds 3 in the paper 2 through the recorder 10. The folds 3 result from using paper 2 prefolded at single page intervals to allow the paper to be conveniently stored and to lay flat after printing. As shown in FIG. 3, as the folds 3 pass through a paper guide 14 in the direction of arrow A, the lack of tension in paper 2 allows the paper near the fold 3 to bow away from and out of contact with the inner guide 16.
Inner guide 16 includes guides extending along the path of the paper 2 through the paper guide 14. The guides extend along the side edges of the paper 2 and are precisely made to be separated by the width of the paper 2 plus a nominal tolerance. The guides are preferably the side walls themselves of the paper guide 14 but may be channels recessed in the inner guide 16. The side edges of paper 2 contact the edges of the guides in inner guide 16. This contact between the edges of the guides or channels and the edges of paper 2 holds paper 2 in position on inner guide 16. When paper 2 is not in contact with inner guide 16, the guides of inner guide 16 cannot precisely position the paper 2. This leads to misalignment of the paper 2.
One solution to the misalignment problem caused by lack of tension on the paper has been to place a weight on the paper being input to the recorder. One embodiment of this type has the weight at the end of a swing arm connected to a pivot. The weight pivots on the swing arm around an attachment point under the force of gravity and contacts the paper, pinching the paper between the weight and a support surface. The weight applies tension to the paper through frictional contact with the paper as the paper is moved through the recorder.
This pivoted weight is typically located on the input side of the paper guide while a drive mechanism for moving the paper through the recorder is typically located on the opposite or output side of the paper guide. When the paper is pulled through the recorder by such a drive mechanism, the tension imparted to the paper by the pivoting weight causes the paper to come into taught contact with an inner guide surface, having guides or channels, within the paper guide prior to the paper being passed to the print head. The guides or channels of the inner guide surface then position the paper prior to being presented to the print head. While this type of tension creating device works, it is costly to manufacture, cumbersome to operate and prone to breakdown.
Another problem commonly occurring with prior art chart recorders is that they must be manually loaded with paper prior to their useful operation. Typically, this loading involves precisely positioning paper manually at an inlet to the chart recorder and thereafter manually moving the paper through the paper guide into position at the print head in order to receive printing. This process is often tedious and time-consuming which causes aggravation, frustration and annoyance to the users of such recorders which often results in inaccurate alignment of the paper. Improper alignment of the paper during the loading process is a leading cause of misalignment of the paper at the print head.